Atlea and Beyond
The Blood War
The Long War between devils and demons (Baatezu and Tanar’ri). Devils are disciplined warriors, giving them an edge in the war, but demons have incredible numbers and ferocity on their side. The Blood War was fought on many battlefields, Avernus, the first layer of the Nine Hells (or Baator), The Grey Waste and Pazunia, the first layer of the Abyss. For many thousands and thousands of years, demons railed against the stronger forces of Baator, being slaughtered, then reborn in the Abyss, then growing bigger until they could fight again, then returning to battle. Baatezu did not have this same strength, they lived but one life and still managed to keep the Abyss at bay (to give some perspective on how well-disciplined their army was).
The war was considered to be ended when Asmodeus became a God, but that’s not really true. The war is simply at a stalemate, both forces withdrawing until their opportunity to attack again arises. It does not help that the demons of the Abyss are constantly bickering and infighting (though the Archdevils of Baator aren’t much better, just more subtle).